Large scale molecular dynamics simulations of the homogeneous nucleation of carbon dioxide in an argon atmosphere were carried out at temperatures between 75 and 105 K. Extensive analyses of the...

We studied the homogeneous nucleation of carbon dioxide in the carrier gas argon for concentrations of CO2 ranging from 2 to 39 mole percent using three experimental methods. Position-resolved pressure...

Abstract. Kinetics of collision-sticking processes between vapor molecules and clusters of low-volatility compounds govern the initial steps of atmospheric new particle formation. Conventional non-interacting hard-sphere models underestimate the collision rate by neglecting long-range attractive forces, and the commonly adopted assumption that every collision leads to the formation of a stable cluster (unit mass accommodation coefficient) is questionable for small clusters, especially at elevated temperatures. Here, we present a generally applicable analytical interacting hard-sphere model for evaluating collision rates between molecules and clusters, accounting for long-range attractive forces. In the model, the collision cross section is calculated based on an effective molecule–cluster potential, derived using Hamaker's approach. Applied to collisions of sulfuric acid or dimethylamine with neutral bisulfate–dimethylammonium clusters composed of 1–32 dimers, our new model predicts collision rates 2–3 times higher than the non-interacting model for small clusters, while decaying asymptotically to the non-interacting limit as cluster size increases, in excellent agreement with a collision-rate-theory atomistic molecular dynamics simulation approach. Additionally, we calculated sticking rates and mass accommodation coefficients (MACs) using atomistic molecular dynamics collision simulations. For sulfuric acid, a MAC ≈1 is observed for collisions with all cluster sizes at temperatures between 200 and 400 K. For dimethylamine, we find that MACs decrease with increasing temperature and decreasing cluster size. At low temperatures, the MAC ≈1 assumption is generally valid, but at elevated temperatures MACs can drop below 0.2 for small clusters.

<p>Nucleation of clusters from the gas phase is a widely encountered phenomenon, e.g. regional air quality and global climate are both directly impacted by particle formation from atmospheric trace gases [1]. Still, the underlying out-of-equilibrium dynamics of this process are not well understood. The classical view of nucleation assumes isothermal conditions where the nucleating clusters are in thermal equilibrium with their surroundings. However, as in all first-order phase transitions, latent heat is released, potentially heating the clusters and suppressing the nucleation. The question of how the released energy affects cluster temperatures during nucleation as well as the growth rate remains controversial.</p> <p>To investigate the nonisothermal dynamics and energetics of homogeneous nucleation, we have performed molecular dynamics (MD) simulations of a supersaturated Lennard-Jones (LJ) vapor in the presence of thermalizing carrier gas. In addition, a previous study of homogeneous nucleation of carbon dioxide in argon carrier gas [2] was revisited for temperature analysis of the growing CO<sub>2 </sub> clusters. The results obtained from these simulations are compared against kinetic modeling of isothermal nucleation and the classical nonisothermal theory by Feder <em>et al</em>. [3], which also predicts the existence of cool subcritical clusters, and has been quite controversial.</p> <p>For the studied systems, we find that nucleation rates are suppressed by two orders of magnitude at most, despite substantial release of latent heat. Our analyses further reveal that while the temperatures of the entire cluster size populations are indeed elevated, the temperatures of the specific clusters driving the nucleation flux evolve from cold to hot when growing from subcritical to supercritical sizes. This resolves the apparent contradiction between elevated cluster temperatures and minor nonisothermal corrections to the nucleation rate, both often reported in literature, and is in excellent agreement with the theory of Feder <em>et al</em>. Our findings provide unprecedented insight into realistic nucleation events and allow us to directly assess earlier theoretical considerations of nonisothermal nucleation.</p> <p> </p> <p>References</p> <p>[1] M. Kulmala <em>et al.,</em> Direct observations of atmospheric aerosol nucleation. <em>Science</em> 339, 943–946 (2013).</p> <p>[2] R. Halonen <em>et al</em>., Homogeneous nucleation of carbon dioxide in supersonic nozzles II: Molecular dynamics simulations and properties of nucleating clusters<em>. Phys. Chem. Chem. Phys.</em> 23, 4517–4529 (2021).</p> <p>[3] J. Feder, K. C. Russell, J. Lothe, G. M. Pound, Homogeneous nucleation and growth of droplets in vapours. <em>Adv. Phys</em>. 15, 111–178 (1966).</p>

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